Instruments and controls

The original purpose of installing instrumentation and control (I C) systems in processing plants was to increase productivity by maintaining operation within established parameters with little or no human intervention. It was assumed that eliminating the human factor would eliminate the chance of operator error. As process plants increased in complexity, more sophisticated control systems were required to maintain productivity. The demands placed on I C systems thereby increased. This created new and sometimes unanticipated reliability and safety problems. 

Understanding the terminology and documentation is very important. ISA S51.1 is a useful general reference ISA S5 explains the t)q)es of drawings used to describe the control systems and ISA S20 gives examples of the standard forms used to specify instrumentation and controls. These three standards serve as a basis for well designed and safe process control systems. 

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Monitoring and control of the production process will be performed by a combination of instrumentation and control equipment plus manual involvement. The level of sophistication of the systems can vary considerably. For example, monitoring well performance can be done in a simple fashion by sending a man to write down and report the tubing head pressures of producing wells on a daily basis, or at the other extreme by using computer assisted operations (CAO) which uses a remote computer-based system to control production on a well by well basis with no physical presence at the wellhead. 

The papers on NDT issues are also published by the Defectoscopiya , Instruments and Control Systems, Oil and gas, Shipbuilding and other journals... 

IControl Systems), Chilton Company, Radnor, Pa., pubHshed monthly. 

Computer Integrated Manufacturing, Instrumentation, and Controls. Large food processing firms are exploring the use of... 

Process Instrumentation and Control Systems. Investment for instmmentation and control systems and their installation typically range between 3 to 10% of the total installed cost for a grassroots continuous process faciUty. Instmmentation and control systems also represent a substantial percentage of the overall faciUty maintenance (qv) costs. Investment costs may be placed in one of two categories, ie, nondiscretionary and discretionary. 

CHOPEY Instrumentation and Process Control CONNELL Process Instrumentation Applications Manual CONSIDINE Process/Industrial Instruments and Controls Handbook, Fourth Edition... 

Electrical costs involve four main components (1) power wiring, (2) lighting, (3) transformation and service, and (4) instrument and control wiring. A breakdown of these component costs as a percentage of total elec trical cost is given in Table 9-60. 

A wet-process plant maldug cement from shale and hmestoue has been described by Bergstrom [Roc/c Prod., 64—71 (June 1967)]. There are separate facilities for grinding each type of stone. The ball mill operates in closed circuit with a battery of Dutch State Mines screens. Material passing the screens is 85 percent minus 200 mesh. The entire process is extensively instrumented and controlled by computer. Automatic devices sample crushed rock, slurries, and finished product for chemical analysis by X-rav fluorescence. Mill circuit feed rates and water additions are governed by conventional controllers. 

Detonation arresters are typically used in conjunction with other measures to decrease the risk of flame propagation. For example, in vapor control systems, the vapor is often enriched, diluted, or inerted, with appropriate instrumentation and control (see Effluent Disposal Systems, 1993). In cases where ignition sources are present or pre-dic table (such as most vapor destruct systems), the detonation arrester is used as a last-resort method anticipating possible failure of vapor composition control. Where vent collec tion systems have several vapor/oxidant sources, stream compositions can be highly variable and... 

Review of planned operation of process, especially the possibility of upsets, modes of failure, unexpec ted delays, redundancy of equipment and instrumentation, critical instruments and controls, and worst-credible-case scenarios... 

The fact that batch processes are not carried out at steady state conditions imposes broad demands on the control system. The instrumentation and control system have to be selected to provide adequate control for a wide variety of operating conditions and a wide variety of processes. In addition, basic process control and shutdown systems have to deal with sequencing issues. This chapter presents issues and concerns related to safety of instrumentation and control in batch reaction systems, and provides potential solutions. 

Listed below are some process safety practices which can help reduce accidents due to instrumentation and control systems. 

Process equipment function changes with different steps in process sequence (e.g., same vessel used as feed tank, reactor, crystallizer pump used to pump in/out). Instrumentation and controls not kept in phase with the current process step (e.g., control set points, interlocks etc.). 

ANSI/IEEE-3.36/1991 Standard Installation, Inspection and testing requirements for power instrumentation and control equipment at nuclear facilites ... 

Instrument and control transformers application and selection 15/495 Relevant Standards... 

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